SAO PAULO, Brazil - A little-known virus spread by mosquitoes is causing one of the most alarming health crises to hit Brazil in decades, officials here warn: thousands of cases of brain damage, in which babies are born with unusually small heads.
Many pregnant women across Brazil are in a panic. The government, under withering criticism for not acting sooner, is urging them to take every precaution to avoid mosquito bites. One official even suggested that women living in areas where mosquitoes are especially prevalent postpone having children.
"If she can wait, then she should," said Claudio Maierovitch, director of the department of surveillance of communicable diseases at Brazil's health ministry.
The alarm stems from a huge surge in babies with microcephaly (my-kroh-SEF-uh-lee), a rare, incurable condition in which their heads are abnormally small. Brazilian officials have registered at least 2,782 cases this year, compared with just 147 in 2014 and 167 the year before.
A woman in Brazil holds her daughter, who was born with microcephaly. An increase in the disease - a form of brain damage - has been blamed on the Zika virus.Global Health: Zika Virus, a Mosquito-Borne Infection, May Threaten Brazil's NewbornsDEC. 28, 2015
At least 40 of the infants have recently died, and some Brazilian researchers warn that cases could multiply in the months ahead. Those babies who survive may face impaired intellectual development for the rest of their lives.
Brazilian researchers say that an obscure mosquito-borne virus that made its way to the country only recently - Zika - is to blame for the sudden increase in brain damage among infants.
But other virologists caution that more testing is needed to prove the dangerous link between the virus and brain damage, leaving the full extent of the threat to the country, and the hemisphere, unclear.
"Why this may have happened in Brazil and not elsewhere is at this stage difficult to answer," said Alain Kohl, a virologist at the University of Glasgow who studies Zika.
"Perhaps it was never properly registered in other areas, or the situation in Brazil is indeed different," he added, citing the possibility that the link between Zika and microcephaly could be related to particular strains of the virus.
The Zika virus has already reached several countries in Latin America, including Mexico, and the Centers for Disease Control and Prevention warns that it could spread in parts of the United States as well. There have already been cases diagnosed in the United States, in travelers who visited affected countries, and the C.D.C. expects these instances to increase.
"I cried for a month when I learned how God is testing us," said Gleyse Kelly da Silva, 27, a toll road attendant in the city of Recife in northeast Brazil, describing how an ultrasound exam had detected microcephaly in the seventh month of her pregnancy with her daughter, Maria Giovanna, born in October.
Just a few months earlier, Ms. da Silva had sought medical attention after experiencing some of Zika's symptoms: fever, joint pain and a red rash.
"I had never heard of Zika or microcephaly," said Ms. da Silva, the mother of three other children. "Now I just pray that my daughter can endure life with this misfortune."
No one knows precisely when the Zika virus made the leap to Brazil from its place of origin in Africa. Some researchers say it could have arrived during the 2014 World Cup, when Brazil welcomed travelers from around the globe. Others think the virus may have come during a canoe race weeks later, when paddlers from French Polynesia, the site of a recent Zika outbreak, arrived in Rio de Janeiro.
Researchers, alert to the rapid increase in cases, say that Zika's spread to Brazil reflects how easily viruses are jumping from one part of the planet to another.
They are particularly worried that the disease is wreaking havoc in a region where the population has not encountered it before, and that climate change may be allowing viruses like Zika to thrive in new domains.
The Brazilian government has stopped short of officially advising women not to get pregnant, but confusion and fear are spreading along with the virus.
"The situation is incredibly frightening," said Andreza Mireli Silva, 22, a worker in a shoe factory in Sergipe State in northeast Brazil who is seven months pregnant. She said she was trying to avoid mosquito bites by wearing long pants despite the heat of the Southern Hemisphere summer and applying insect repellent every three hours.
Zika, named for the forest in Uganda where scientists discovered it in the 1940s, often goes unnoticed in the people it infects and was not considered especially life-threatening before spreading to Brazil.